Rain sensor

ABSTRACT

In a moisture sensor (1), the effective sensor field (4) is formed by two sensor elements (8a, 8b,) having interdigitating parallel tracks. These sensor elements consist of layers of a metal nitride compound, especially a chromium-nitrogen compound, deposited by means of magnetron cathode sputtering or glow discharge onto a dielectric substrate (2), the metal nitride layers being deposited onto the substrate surface which has first been cleaned by ion sputtering. These sensor element layers (8a, 8b) have a high hardness of about 2,000 on the Vickers scale, as a result of which the sensor elements (8a, 8b) are highly resistant to abrasive wear such as that caused by windshield wipers or scratching from ice. The moisture sensor can be produced with a low electrical surface resistivity of &lt;100 ohms□ and is chemically stable with respect to environmental effects such as salt water and/or windshield cleaning fluids. As a result, the proposed moisture sensor is especially suitable for use as a rain sensor on vehicles, the rain sensor preferably being mounted on a window of the vehicle.

This application is a divisional of Application Ser. No. 08/490,725filed June. 15, 1995; now U.S. Pat. No. 5,653,855.

BACKGROUND OF THE INVENTION

The invention pertains to a process for the production of a moisturesensor of the type comprising two electrically conductive elementsarranged on a dielectric substrate to form a sensor field whichundergoes a change in impedance when wet.

Moisture sensors of this type and also methods for their production arealready known. For example, in EP 0 507 491 A2, a process for theproduction of a rain sensor installed on a vehicle window is described.This sensor consists essentially of two electrically conductive surfacesintermeshing like fans, which are attached to a glass plate to form aresistance element. The change in the impedance of this resistanceelement caused by the presence of moisture on the surface of the sensoris recorded electronically and forms the sensor output variable. The twoconductive surfaces consist of a metal oxide layer, which is depositedpyrolytically on the glass substrate. After the forming of the glasssubstrate, the final surface structure is produced by means of, forexample, laser etching or a chemical etching process based on the use ofa photomask.

A disadvantage of moisture sensors produced in this way is that theconductivity of the metal oxide layer, e.g., a layer of tin oxide, issignificantly reduced by the forming process, normally carried out atabout 600° C., and by the following process of tempering the glassplate, and the sensitivity of the sensor is reduced.

SUMMARY OF THE INVENTION

The invention is based on the task of creating a moisture sensor of thetype described above which does not suffer from the disadvantagesindicated above and which, in addition, adheres very strongly to thesubstrate, is highly resistant to the abrasive wear, and is alsochemically stable with respect to environmental influences. A furthertask of the invention is to provide a process for producing a moisturesensor of this type.

For a moisture sensor according to the invention, the conductivesurfaces (sensor elements) are deposited under vacuum onto the substratesurface (glass surface), which has been cleaned by ion bombardment andthus activated for the following coating step. The structure of thesensor field, consisting of at least two sensor elements, is produced bymeans of a photomask technique. For this purpose, a photoresistive layeris applied to the dielectric substrate, dried, and covered by aphotomask corresponding to the sensor structure. The photoresist isexposed to light and then developed. The unexposed parts of thephotoresist are dissolved away by the developer, and thus a negativeimage of the sensor field is obtained. The subsequent cleaning of thebared substrate areas by means of ion bombardment offers the advantagethat loosened impurities on the substrate can be completely removed, andimpurities which could recontaminate the surface of the substrate beforethe next process step, i.e., the application of the conductive layer,are prevented from settling back on the substrate. In addition, the ionbombardment has the advantageous effect of activating the surface of thesubstrate to be coated, which helps to increase strength with which theconductive layer adheres to the substrate.

According to the invention, this ion-sputtering cleaning process can becarried out either by means of a glow-discharge in a noble gas, andpreferably argon and/or O₂ gas or by means of a high-frequencysputter-etching process. The process parameters to be used in thesecases have the following values:

P_(argon) =2×10⁻³ mbar;

P_(HF) =900 W;

U_(DC) =550 V; and

t=60 sec.

The sensor-forming layer to be applied after the cleaning of thenegative substrate structure is preferably deposited by reactivemagnetron sputtering onto the substrate. A metal nitride compound,preferably a chromium-nitrogen compound, is selected as thesensor-forming layer. Reactive DC magnetron sputtering of CrN_(x) in anargon/nitrogen atmosphere has been found especially advantageous. The DCmagnetron sputtering process can be carried out advantageously with thefollowing process parameter values:

P_(sputtering) =6×10⁻³ mbar;

P_(DC) =4 kW;

t=20 sec.

The chromium nitride layer thus produced contains chromium and nitrogenin a ratio of 55:45%; the layer of d_(CrNx) is 100 nm thick; and thesurface resistivity R.sub.□ is 50 ohms/□. By the use of other values forthe process parameters, it is possible to deposit layers in thicknessesranging from 20 to 500 nm, and preferably 100 nm, onto the substrate.For the electrical surface resistivity R.sub.□, values of less than 300ohms/□, preferably values of less than 100 ohms/□, are suitable.Decreasing the electrical surface resistivity has the advantage ofincreasing the sensitivity of the sensor. It is advantageous for themetal nitride layer also to contain oxygen.

In the final step of the process, the exposed photoresist layer stilladhering to the substrate (preferably a glass substrate) is removed by awet chemical treatment.

The chromium nitride layers deposited in this way on the glass substrateare surprisingly very hard, offering a hardness of about 2,000 on theVickers scale, and are chemically stable with respect to environmentaleffects, especially those exerted by air pollution, salt water,alternating temperature stresses, and glass cleaners. In combinationwith the high resistance to abrasive wear imparted by its hardness, amoisture sensor of this type is suitable especially for use as a rainsensor on the outside surface of a vehicle window exposed to severeabrasive wear from the effects of windshield wipers, from scratching byice, and from impacts by various types of hard particles.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic plan view of the sensor according to theinvention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A moisture sensor 1, shown in FIG. 1, consists essentially of two flatsensor elements 8a, 8b, which are mounted on a common dielectricsubstrate 2. The individual sensor elements 8a, 8b consist of parallelconductive tracks 10, 10', 10", 10'", . . . , each of which is connectedconductively at its end to an electric terminal 6a, 6b. The free ends ofconductive tracks 10, 10', 10", 10'", . . . of individual sensorelements 8a, 8b interdigitateconductive tracks 10, 10", . . . of sensorelement 8a alternating in parallel fashion with conductive tracks 10',10'" of sensor element 8b. When sensor field 4 is wetted by, forexample, a drop of water, the electrical resistance between sensorelements 8a and 8b changes. This change in impedance can be recorded bya suitable electronic sensor unit (not shown in FIG. 1) connected toelectrical terminals 6a, 6b and can be used for a subsequent controlpurpose such as for turning on a windshield wiper motor.

What is claimed is:
 1. Moisture sensor comprising a dielectricsubstrate, and two electrically conductive sensor elements arranged onsaid substrate adjacent to each other but insulated from each other,said sensor elements comprising a metal nitride layer.
 2. Moisturesensor as in claim 1 wherein said metal nitride is chromium nitride. 3.Moisture sensor as in claim 1 wherein said metal nitride containsoxygen.
 4. Moisture sensor as in claim 1 wherein said metal nitridelayer is 20-500 nm thick.
 5. Moisture sensor as in claim 4 wherein saidmetal nitride layer is 100 nm thick.
 6. Moisture sensor comprising adielectric substrate, and two electrically conductive sensor elementsarranged on said substrate adjacent to each other but insulated fromeach other, said sensor elements comprising a metal nitridelayer,wherein said metal nitride layer has a surface resistivity of lessthan 300 ohms/□.
 7. Moisture sensor as in claim 6 wherein said metalnitride layer has a surface resistivity of less than 100 ohms/□. 8.Moisture sensor as in claim 1 wherein said substrate is glass. 9.Moisture sensor as in claim 1 wherein each sensor comprises a pluralityof parallel tracks which interdigitate with parallel tracks of the othersensor.
 10. Moisture sensor as in claim 1 wherein said metal nitridelayers of said electrically conductive sensor elements each have arespective exposed electrically conductive surface disposed so thatmoisture contacting the moisture sensor engages the exposed surfaces andprovides an electrical connection therebetween.